Humans have wondered throughout history how we are able, through the sense of vision, to discern the nature and color of objects far removed from our bodies - objects with which we are obviously not in contact. The ancient Greek philosophers correctly reasoned that something must pass between our eyes and the objects we see. Plato developed an emanation theory of vision, which says that an inner fire gives rise to visual rays shooting outward from the eye. Other philosophers either accepted the emanation theory or used some other approach to explain human vision.
During the middle ages, Arab natural philosopher Alhazen rejected the emanation theory of vision. He was convinced that an optical image similar to the one produced by a pin-hole camera is produced in the eye.
During the Italian Renaissance, the great scientist-painter Leonardo da Vinci developed perspective drawing, and speculated about human vision. He was convinced that there is some kind of image inside the eye. As the laws of light refraction and the nature of light were unknown then, da Vinci could not develop his theory successfully.
The seventeenth century marks the start of the modern era for the study of light and vision. Spectacle lenses had been discovered by 1285, and positive lenses have been used to improve the performance of the pin-hole camera. Kepler understood how the lenses worked in the telescope he was using. He correctly believed that there is also some kind of retinal image in our eye, but the vision in its totalty was still not clear to him.
Throughout the period of history described so far, nothing was known about the physical nature of light. Isaac Newton was the first who realized that white light is composed of the whole spectrum.
Nowadays, it is well known that light is electromagnetic radiation. The electromagnetic spectrum extends from very low frequency radio waves, through microwaves, infrared, visible and ultraviolet light to x-rays and gamma rays. Our eyes respond to the visible light. If we want to detect the rest of the electromagnetic spectrum, special instruments ranging from radio receivers to scintillation counters are required. An exact description of electromagnetic radiation requires a thorough knowledge of quantum electrodynamics and Maxwell's electromagnetic field equations which is beyond the scope of this work. The visible spectrum is considered to have the wavelengths between 380 and 770 nm. It means if electromagnetic radiation of such wavelength hits our eye, we will see it. The perceived color depends on the wavelength of the radiation. We will deal with measuring light independently from the wavelength first.